Lithium silicate glass ceramics are used in dentistry, in particular for the preparation of dental crowns and small bridges, because of their high translucence and very good mechanical properties. The known lithium silicate glass ceramics usually contain SiO2, Li2O, Al2O3, Na2O or K2O and nucleating agent such as P2Os as main components.
EP 0 916 625 A1 describes translucent lithium disilicate glass ceramic products which can be prepared as blanks and can be processed, in particular by plastic deformation under the action of pressure and heat or machining, to form shaped translucent dental products with high strength. To prepare the lithium disilicate glass ceramic products, firstly a melt of a starting glass is produced which contains the components SiO2, Li2O, La2O3 and/or Al2O3 and MgO and/or ZnO. This melt is shaped and cooled in a suitable manner and subjected to at least one heat treatment in order to obtain a glass ceramic product in the form of a blank. In order to match the colour of the glass ceramic products to the colour of natural tooth material, the starting glass can furthermore have colour and fluorescence components which are preferably selected from the group consisting of CeO2, V2O5, Fe2O3, MnO2, TiO2, Y2O3, Er2O3, Tb4O7, Eu2O3, Yb2O3, Gd2O3, Nd2O3, Pr2O3, Dy2O3, Ag2O, SnO2 and Ta2O5.
EP 1 505 041 A1 describes lithium metasilicate glass ceramics which are processed, in particular by means of CAD/CAM methods, to form dental restorations and can be converted into high-strength lithium disilicate glass ceramics by a subsequent heat treatment. To prepare the glass ceramics, firstly a melt of a starting alas is formed which contains SiO2, Li2O, Al2O3, K2O and a nucleating agent such as P2O5 as main components. The melt of the starting glass is shaped and cooled in a suitable manner and subjected to two heat treatments in order to obtain a glass ceramic product in the form of a blank. The starting glass can have, among other things, colouring and fluorescent metal oxides. The metal is preferably selected from the group consisting of Ta, Tb, Y, La, Er, Pr, Ce, Ti, V, Fe and Mn, wherein in the examples the oxides TiO2, V2O5, Fe2O3, MnO2, CeO2, Y2O3, La2O3, Ta2O5, Tb4O7 and Er2O3 are used. Similar lithium silicate glass ceramics are described in EP 1 688 398 A1.
From W. Buchalla, “Comparative Fluorescence Spectroscopy Shows Differences in Non-Cavitated Enamel Lesions”, Caries Res. 2005, 39, 150-156, it is known that natural teeth display a bluish-white fluorescence with wavelengths in the range of from 400 to 650 nm under ultraviolet light.
Rukmani et al., J. Am. Ceram. Soc. 2007, 90, 706-711, describe the influence of V and Mn colorants on the crystallization behaviour and the optical properties of Ce-doped lithium disillate glass ceramics. To prepare the glass ceramics, a mixture of the starting materials SiO2, ZrO2, Li2CO3, K2CO3, MgCO3 and Al(PO3)3 is mixed with CeO2, V2O5 and MnO2, the mixture is melted at 1500° C. in platinum crucibles, cooled and then subjected to several heat treatments in a tube furnace with air supply.
However, it has been shown that the lithium silicate glass ceramics known from the state of the art have insufficient fluorescence properties and cannot imitate the fluorescence properties of natural tooth material to a sufficient extent, in particular under UV light. Dental restorations prepared from such glass ceramics thereby become recognizable as restorations, in particular under the influence of CV light, or are perceived as tooth gaps or defects.